There is a current trend to integrate photonic devices and electronic devices on the same substrate. A silicon-on-insulator (SOI) substrate can be used as the supporting substrate for such integration. When optical waveguides are formed a cladding is provided around the core of the waveguide for confining a light wave propagated along the waveguide. The core material has an index of refraction which is larger than that of the cladding. If silicon is used as the core material of a waveguide, having an index of refraction of about 3.47, the waveguide cladding can be formed of silicon dioxide which has an index of refraction of about 1.54. When a silicon-on-insulator substrate is used as the supporting substrate, the cladding material below the waveguide core can be the buried oxide (BOX) insulator of the SOI substrate, which is again typically silicon dioxide. The BOX cladding can also function to prevent optical signal leakage by evanescent coupling from the silicon waveguide core to a supporting silicon of the SOI structure. However, to prevent such evanescent coupling, the BOX cladding material beneath the waveguide core must be relatively thick, for example, greater than 1.0 μm and often 2.0 μm-3.0 μm thick. When the Box cladding material is thick it inhibits heat flow to the underlying silicon, which can act as a heat dissipator. In addition, when certain electronic devices, such as high speed logic circuits, are integrated on the same SOI substrate as photonic devices, the BOX of the SOI substrate must be relatively thin, typically having a thickness in the range of 100-200 nm. Such a thin BOX insulator, while providing a good substrate for the electronic devices, is insufficient to prevent optical coupling of the silicon waveguide core to the underlying supporting silicon of the SOI substrate, which causes undesirable optical signal loss.
One way to prevent evanescent coupling of a silicon waveguide core to supporting silicon of a substrate is discussed in U.S. Pat. No. 7,920,770. There, an etched cavity is formed in the silicon support material at an area below a buried insulator. The cavity serves to increase the distance between the waveguide core and the supporting silicon. The cavity may remain empty or be filled by a gas or other material having refractive properties which prevent the silicon waveguide core from easily optically coupling to the cavity material or silicon in which the cavity is formed. The cavity may be formed after a waveguide is formed by beginning an etch of the supporting silicon at an area outside the area of the waveguide core. The etch process produces a cavity in the supporting silicon which expands downwardly and outwardly of the etch location. This produces a large cavity which may encompass areas of the silicon substrate which are not below the waveguide and not needed for optical isolation. In addition, the cavity may be formed below photonic devices which are coupled to the waveguide such as an optical modulator connected to the waveguide. If the optical modulator or other photonic device coupled to the waveguide is operated in a manner which generates or requires the addition of heat during operation, the cavity and/or material within the cavity disrupts heat flow to the supporting silicon substrate material to lessen its effectiveness as a heat sink.
Accordingly, another method and structure for forming a silicon-on-insulator structure which has a relatively thin BOX insulator and which is capable of optically decoupling the waveguide core from the substrate material is desirable.